The present invention relates to improvements in the accurate monitoring of fluid flow by way of thermal exchanges which are uniquely coordinated to suppress lags in response and which advantageously lend themselves to the characterization of mass rate of flow, and, in one particular aspect, to novel, improved and relatively uncomplicated thermal mass flowmetering in which multiple-stage sequenced controlled heatings of a flowing medium automatically condition it to promote exceptional speed of response as well as precision and stability in measurements developed in respect of energy supplied to one of the stages.
Metering of fluid flow may be accomplished in a variety of ways, with the selection of a particular approach commonly being determined by such factors as the composition, volume and flow rates of the fluent materials to be encountered, and by the bulk, costs and power requirements of equipment which will offer desired reliability and precision in the expression of output measurements and/or control in specified terms. For some purposes, only mere rough characterizations of flow may be enough, and fluid-induced movements of relatively simple ball or rotor elements may be viewd or otherwise sensed to provide such information. Positive-displacement pump-type units can be used where volume is the parameter of interest, as can certain vaned-rotor devices, and mass can be calculated with the aid of corrections for density if temperature and viscosity problems are resolved. Pressure-responsive devices, of the venturi, nozzle, orifice and Pitot-tube types, are robust and inexpensive and have been widely used also. Mass rate of flow, and integrations of same into total mass flow, are data which have especially-significant relations to what may be involved in chamically-reacting or energy-dependent processes, or in other supply or dispensing of fluents, and among the efforts to obtain such data directly have been the now well-known angular-momentum devices, such as those involving cooperating impeller and reaction-turbine elements.
Among the classes of instruments which have been popular for gaseous-fluid measurements, in particular, are so-called "hot wire" flowmeters which exploit the principle that a flowing fluid encountering a mass at high temperature tends to cool it to an extent which at least in part depends upon the rate of flow. By way of example of one form of such a thermal flowmeter, it has been known to introduce an electrical heater into a stream, with its supply of electrical power being kept the same, and to interpret the differences between upstream and downstream temperatures as measurements of flow. However, heaters and temperature detectors disposed within a flow stream can upset laminar flow conditions, and such measurements tend to be highly non-linear. Also known is the metering of mass flow by determining temperature differentials between temperature-sensitive heating coils which are in upstream and downstream relations to the fluid and which are elements of electrical bridge circuitry serving both measuring and heating purposes; such flow has been carried in laminar-flow by-pass tubing , with insulation encasing the wound tubing (U.S. Pat. Nos. 3,938,384 and 3,851,526). Recent patent discussions of thermal flowmeters have included that in U.S. Pat. No. 4,297,881, wherein fixed-temperature and fixed temperature difference types of apparatus are distinguished, and wherein related electrical bridge circuitry has been described. And, in U.S. Pat. No. 4,300,391, hot-wire anemometers and problems commonly associated with them are reviewed. Probe-mounted wire-type resistances for mass flow measurements appear in U.S. Pat. No. 4,304,128, and silicon resistances and thin-film metal resistances are involved in the flowmeters of U.S. Pat. Nos. 4,319,483 and 4,320,655, for example.